Ultrasonic flow measurement installation

ABSTRACT

An ultrasonic fluid-flow measuring device in which a measuring tube having a rectangular cross section and a measuring tube wall lying opposite a pair of ultrasonic transformers is provided. The device utilizes three flat surface elements angled with respect to each other to reflect an ultrasonic beam so as to guide its passage through the fluid. Such structure provides a device in which the flow of the fluid is not impaired at any place in the measuring tube, and in which the inner walls of the measuring tube are free of niches which might promote the deposition of particles located in the fluid.

The invention concerns an ultrasonic flow measuring device according tothe echo time method having a measuring tube traversed by the flow of amedium, whereby the measuring tube has two ultrasonic transformersdesigned as combined transmit and receive transformers stagger-mountedin the direction of flow of the medium and at least one reflectorreflecting the ultrasound between the ultrasonic transformers.

This kind of an ultrasonic flow measuring device is known e.g. from theteachings of DE-A1-35 39 948. In the known arrangement, a medium flowsthrough the measuring tube, which has a fundamentally rectangular innercross-section. Two ultrasonic transformers are stagger-mounted in thedirection of flow on two opposing, parallel measuring tube walls. Twocombined transmit and receive transformers, which are designed asinterdigital transformers according to DE-A1 30 20 282, serve thepurpose as ultrasonic transformers.

Interdigital transformers have a direction of radiation or reception,respectively, lying at an angle to the surface. The ultrasound radiatedfrom a first ultrasonic transformer is transmitted to a first reflector,which is mounted in the direction of flow on the measuring tube walllying opposite the first ultrasonic transformer and staggered with thefirst ultrasonic transformer. From there, the ultrasound is reflected toa second reflector which is mounted in the direction of flow on themeasuring tube wall lying opposite the first reflector and is staggeredwith the first reflector. The second reflector directs the ultrasound tothe second ultrasonic transformer. The flow velocity of the mediumflowing in the measuring tube is in principle detected in that onemeasures the differences of the echo times of ultrasonic waves in andagainst the direction of flow. The difference of the echo times therebyserves as a measure for the flow velocity in the measuring tube.

The disadvantage of the known arrangement is that interdigitaltransformers are costly to manufacture and therefore expensive. Afurther disadvantage consists in that the inner walls of the measuringtube in the region of the reflectors are coated with anultrasound-reflecting material or, respectively, outside of this regionwith an ultrasound-absorbing material.

According to a specific embodiment according to DE-A1-28 28 397, twoultrasonic transformers are mounted at an angle to the measuring tubewall and in a direction towards each other, and are staggered in thedirection of flow on opposing measuring tube walls of the measuring tubethrough which a medium is flowing. In this manner, both ultrasonictransformers are designed as combined transmit and receive transformershaving a transmit and receive direction perpendicular to the surface.The ultrasonic transformers are operated alternately as a transmittingtransformer or a receiving transformer, respectively. Thus, ultrasonicwaves are transmitted in and against the direction of flow and withoutinterconnection by means of reflectors directly to the respectiveultrasonic transformer lying opposite. The adjusting echo timedifference serves as a measure for the flow velocity of the mediumflowing in the measuring tube.

The disadvantage of this known specific embodiment is that theultrasonic transformers must be mounted at an angle to the surface ofthe measuring tube and thus to the direction of flow. Therefore,resistances to flow arise at the location of the mounting of theultrasonic transformers either in that the ultrasonic transformers jutout into the measuring tube and thus into the flow, or in that theultrasonic transformers are mounted set back from the measuring tubewall and thus form niches. In both cases, changes are caused in the flowwhich can lead to deposits on the ultrasonic transformers. In addition,eddies can arise as a function of the velocity of the flowing medium,which falsify the measuring signal.

It is the task of the invention to design a measuring tube such that onecan dispense with the use of expensive interdigital transformers,whereby it should be possible to incorporate the ultrasonic transformerin the measuring tube wall without niches being formed.

This task is solved according to the invention in that the ultrasonictransformers have transmit and receive directions lying perpendicular totheir surface; that both ultrasonic transformers are mounted on astraight wall of the measuring tube in the direction of flow; that thetransmit and receive directions of the ultrasonic transformers areperpendicular to the direction of flow; that the measuring tube wall hasa chamfer in each region lying directly opposite the ultrasonictransformers so that the ultrasound radiated from each ultrasonictransformer is reflected off of these chamfers onto the region of themeasuring tube wall between both of the ultrasonic transformers and fromthere off of the additional chamfer onto the additional ultrasonictransformer.

Through the invention, the ultrasound is transmitted and received bymeans of triple reflection perpendicular to the direction of flow andthus to the measuring tube wall. Thus, the inexpensive ultrasonictransformers, send and receive perpendicular to the surface, can be usedadvantageously, and an installation is possible plane- parallel to themeasuring tube wall.

The ultrasonic transformers are mounted on one side of the measuringtube so that the evaluating electronics can be set up on this side andthe connecting lines remain short.

The invention makes possible the installation of the ultrasonictransformers flush to the inner wall of the measuring tube, i.e. alow-loss transmission of the ultrasound through the measuring tube wall.By this means the flow rate profile in the measuring tube is not changedand by this means there is no falsification of the measured value.Moreover, a niche formation and thus deposits are excluded. Thus, themeasuring tube is suitable for use in the food industry, where due tothe danger of the formation of germs, emphasis is placed on theavoidance of deposits. Through the refinement of the measuring tubewall, the ultrasound is supplied with low loss: the separate reflectorscan be dispensed with and a one-piece design is possible, whereby thecost of production is reduced considerably. Moreover, the medium flowingthrough the measuring tube can be channeled more flow-effectively.

The wall thickness d₁ of the measuring tube is advantageously chosen inthe reflecting areas so that the ultrasound reflection is maximumaccording to the formula d₁ = (2n-1) λ/4 with n=1,2,3 . . . ,λ=shockwave length in the housing wall. By this means a lowest- losspossible supply of ultrasound is guaranteed.

It is advantageous when each ultrasonic transformer is set up on theoutside of a measuring tube wall so that the ultrasound is transmittedor received through the measuring tube wall and that the wall thicknessd₂ of the measuring tube in the region of the ultrasonic transformer isselected so that the ultrasound transmittance is maximum according tothe formula d₂ =n. λ/2, with n=1,2,3 . . . , λ=the shockwave length inthe housing wall. A low-loss ultrasonic coupling and decoupling throughthe measuring tube wall is made possible by this means.

An advantageous further development consists therein that the ultrasonictransformers are respectively equipped with a high-grade steel base;that one flange per ultrasonic transformer is provided in the measuringtube wall, into which the ultrasonic transformer, respectively, is ableto be screwed flush to the inner wall of the measuring tube.Consequently, the ultrasonic transformers are easily exchangeable andare resistent to chemically aggressive media due to the high-grade steelbase.

Exemplified embodiments of the invention are described in light of theFIGS. 1 and 2 in the following. Therein are shown:

FIG. 1 an ultrasonic measuring tube in longitudinal section, in whichthe ultrasonic transformers are mounted onto a measuring tube wall; and

FIG. 2 an ultrasonic measuring tube in longitudinal section in whose onemeasuring tube wall one flange is provided per ultrasonic transformer.

According to the FIGS. 1 and 2, the ultrasonic transformers arestagger-mounted in the direction of flow on a first measuring tube wall1a of the measuring tube 1 with an essentially rectangular innercross-section. The region of the second measuring tube wall 1b of themeasuring tube 1, located directly opposite the ultrasonic transformers2, 3, is designed as chamfers 4, 6, respectively.

Outside of this region, the measuring tube walls 1a, 1b run parallel toeach other. The ultrasonic transformers 2, 3 are designed as combinedtransmit and receive transformers, which have transmit and receivedirections lying perpendicular to their surfaces. The ultrasonictransformers 2, 3 are triggered by means of a drive circuit arrangementso that an alternating transmit and receive mode is set up. Under theappropriate control, a first ultrasonic transformer 2, 3 transmitsultrasound perpendicularly to the direction of flow towards a firstchamfer 4 or 6, respectively, of the second measuring tube wall 1b. Fromthere, the ultrasound is reflected across a region 5 of the firstmeasuring tube wall 1a between the ultrasonic transformers 2, 3 onto asecond chamfer 4 or 6, respectively, of the second measuring tube wall1b, which reflects the ultrasound to the ultrasonic transformer 2, 3,which is operating as a receiver. Subject to the alternating transmitand receive operation of the ultrasonic transformers 2, 3, theultrasound radiated from the ultrasonic transformers is transmittedalternately in and against the direction of flow v.

The echo time differences of the ultrasound resulting from this can beevaluated according to the socalled echo time difference measurementwith the phase difference measurement or as a direct echo-timemeasurement with the "sing-around method". During the echo timemeasurement, the frequency of the ultrasonic transformers, whichtransmit in and against the direction of flow, is set up, respectively,so that the number of the ultrasonic wavelengths along the distancebeing measured is constant. Both methods are described at length inDE-A1-28 28 937.

In order to obtain a best possible reflection of the ultrasonic waves onthe inner wall of the measuring tube 1, the wall thickness d₁ of themeasuring tube 1, at least in the region of the reflection, is to beselected according to the formula d₁ =(2n-1) λ/4 with n=1,2,3 . . . ,λ=acoustic wavelength in the housing wall.

Thereby, according to FIG. 1, the wall thickness d₂ of the measuringtube in the region of the ultrasonic transformers 2, 3 should beselected according to the formula d₂ =n λ/2 with n=1, 2, 3 . . . ,λ=acoustic wavelength in the housing wall, in order to obtain a goodultrasonic transmittance by this means.

According to FIG. 1, a connecting sleeve 8 can be provided,respectively, to connect the measuring tube 1 to an existing tubingsystem which carries the flowing medium. This connecting sleeve 8 isdesigned so that a best possible flow junction of a rectangular tubecross-section to a round tube cross-section or vice-versa is madepossible on the side turned towards the measuring tube.

In an exemplified embodiment according to FIG. 2, the measuring tube isconnected on both faces by one coupling flange 9 each. The region markedwith y is equally designed so that a junction from a rectangular tubecross-section to a round tube cross-section is made possible. Themeasuring tube can be flange-mounted to a tubing system, which carriesthe medium to be measured, by means of the coupling flange 9.

I claim:
 1. An ultrasonic fluid flow measuring device that operatesaccording to the echo-time method, comprising:one axially extendingmeasuring tube having a rectangular inner cross section and a first,straight, measuring tube wall and a second measuring tube wall, thesecond measuring tube wall lying opposite the first measuring tube walland being formed from three flat surface elements angled against eachother;two ultrasonic transformers of the combined transmit and receivetype, said transformers being mounted at axially spaced apart locationsalong the first, straight wall of the measuring tube in the direction offlow of the fluid through the measuring tube, said transformers eachhaving a surface parallel with the first wall and being of the type thatcan both receive and transmit beams of ultrasonic waves orthogonallywith respect to their surfaces so that the ultrasonic beams of thetransmitting ultrasonic transformer are issued into the fluidperpendicular to the direction of flow; one ultrasound reflecting innersurface located on the first measuring tube wall between the twoultrasonic transformers, and one angled ultrasound reflecting surfacelying along each of two of said angled portions of the second measuringtube wall opposite each of the ultrasonic transformers; said angledultrasound reflecting surfaces being tilted towards each other at asharp angle with respect to the direction of flow of the fluid and beingconnected to each other by means of a straight surface element parallelto the direction of flow of the fluid; and whereby the ultrasonic beamemanating orthogonally from one of the respective transmittingultrasonic transformers strikes the opposed angled ultrasound reflectingsurface of the second measuring tube wall, and is reflected in thedirection towards the ultrasound reflecting inner surface of the firstmeasuring tube wall between the ultrasonic transformers, from which itis reflected onto the second angled ultrasound reflecting surface of thesecond measuring tube wall, and from which it is reflected onto therespective opposed receiving ultrasonic transformer.
 2. The ultrasonicflow measuring device according to claim 1, wherein the wall thickness(d₁) of the measuring tube in the region of the reflecting surfaces isselected so as to assure that the ultrasonic beam reflects off of thereflecting surface with maximum intensity by selecting the wallthickness according to the formula d₁ =(2n-1) λ/4, with n=1,2,3 . . . ,and where λ=the shockwave length in the portion of the measuring tubewall off which the beam reflects.
 3. The ultrasonic flow measuringdevice according to claims 1 or 2, characterized in that each ultrasonictransformer is set up on the outside of the first measuring tube wall sothat the ultrasound is transmitted or received through the measuringtube wall and that the wall thickness (d₂) of the measuring tube in theregion of the ultrasonic transformers is selected so that the ultrasonictransmittance is at a maximum according to the formula d₂ =n λ/2, withn=1,2,3 . . . , and where λ=shockwave length in the portion of themeasuring tube wall off which the beam reflects.
 4. The ultrasonic flowmeasuring device according to claim 1 or 2, characterized in that theultrasonic transformers are equipped with a high-grade steel base,respectively, and each ultrasonic transformer is provided with a flangelocated in the measuring tube wall into which the ultrasonic transformermay be screwed until it is generally flush with the inner surface of thefirst measuring tube wall.
 5. The ultrasonic flow measuring deviceaccording to claim 1, wherein the measuring tube is manufactured from amaterial resistant to chemically aggressive media.